1. Field of the Invention
The present invention relates generally to marine seismic prospecting and, more particularly, to a method for using production Dual Sensor seismic data to determine water bottom reflectivity in a surface consistent manner, that is, determining the different values at different locations.
2. Description of the Related Art
In marine seismic prospecting, a seismic survey ship is equipped with at least one energy source and at least one receiver for taking seismic profiles of an underwater land configuration. The act of taking profiles is often referred to as "shooting" or taking "shots" due to the fact that explosive devices had been commonly used for many years as energy sources. An energy source is designed to produce compressional waves that propagate through the water and into the underwater subterranean land formations. As the compressional waves propagate through the subterranean formations, they strike interfaces between formations, commonly referred to as strata, and reflect back through the earth and water to a receiver. The receiver typically converts the detected waves into electrical signals which are later processed into an image that provides information about the structure of the subterranean formations.
Presently, one of the most common marine energy sources is an air gun that discharges air under very high pressure into the water. The discharged air forms an energy pulse which contains frequencies within the seismic range. Another marine energy source which is frequently used is a marine vibrator. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies.
Just as different energy sources may be used to generate acoustic waves in marine applications, different receivers may be used to detect reflected acoustic waves. The receivers most commonly used in marine seismic prospecting are hydrophones. Hydrophones convert pressure waves into electrical signals that are used for analog or digital processing. The most common type of hydrophone includes a piezoelectric element which converts physical signals, such as pressure, into electrical signals. Hydrophones are usually mounted on a long streamer which is towed behind the survey ship at a depth of tens of feet.
Alternatively, marine seismic prospecting may use different types of receivers which detect different characteristics of the environment. For instance, in Dual Sensor bottom cable seismic recording, a combination of pressure sensitive transducers, such as hydrophones, and particle velocity transducers, such as geophones, are deployed on the marine bottom. Geophones are typically used in land operations where metal spikes anchor the geophones to the ground to maintain correspondence of geophone motion to ground motion. In marine applications, however, anchoring the geophones is difficult. Typically, therefore, cylindrical gimbal geophones are attached to the bottom cable. After the cable is deployed from the seismic survey ship, the geophones lie in contact with the marine bottom where they fall. The gimbal mechanism inside the cylinder orients the geophone element vertically for proper operation. Typically, miles of bottom cable are deployed in a planned pattern such as a single line or several substantially parallel lines.
The use of water bottom cables is particularly effective in obtaining full three dimensional coverage in areas too shallow or too congested with obstacles for gathering seismic data with a towed streamer. While the bottom cable technique allows access to areas denied by the towed streamer method, an additional, unwanted "ghost" reflection from the air water interface, along with subsequent reverberations, occurs for each primary reflection wave. The time delay between the primary reflection signal and the ghost reflection signal is greater with the bottom cable method than with the towed streamer method because the detectors are farther removed from the air-water interface, except in shallow water.
Two basic approaches have been proposed for eliminating the ghost reflection. The first approach involves recording signals from detectors at different depths and performing a wavefield separation. The second, and operationally more straightforward, approach, utilizes co-located pairs of pressure and velocity detectors, as in, for example, U.S. Pat. No. 2,757,356, "Method and Apparatus for Canceling Reverberations in Water Layers", issued to Hagarty. This second approach capitalizes upon the fact that pressure and velocity detectors generate signals which are the same polarity for upward travelling waves but are of opposite polarity for downward travelling waves, that is, the ghost reflections. This indicates that the two signals can be properly scaled and summed to eliminate the unwanted reverberations associated with each reflection. In the frequency domain, this relationship expresses itself in the complimentary amplitude spectra of the two sensors. When the signals are properly summed, a smooth amplitude spectrum results.
U.S. Pat. No. 4,979,150, issued to present co-inventor Barr, assigned to the assignee of the present invention, and entitled "Method for Attenuation of Water-Column Reverberations" describes a Dual Sensor bottom cable method for attenuating the unwanted water column reverberations associated with each reflection signal in the seismic data by combining the pressure and velocity signals recorded at each receiver station. Proper combination of the pressure and velocity signals, in order to remove the component of the signal representing energy which is trapped in the water layer, can only be performed after scaling the velocity signal by a scale factor S given by ##EQU1## where R is the water bottom reflectivity. Thus the scale factor requires determining the water bottom reflectivity, which depends upon the acoustic impedance of the bottom material. Since the acoustic impedance of the bottom material, and hence the water bottom reflectivity, can vary among different source and receiver locations, the scale factor can be expected to vary at different locations too. A "surface consistent" map of water bottom reflectivities gives the different values at different locations.
In the past, a calibration survey has been used to estimate the water bottom reflectivity R. In the dual sensor operations described above, an estimate of the water bottom reflectivity is made by collecting separate reference information, generated by shooting a small seismic source directly over the receivers. The collection of this survey data requires additional time and cost beyond the data acquisition phase of the survey.
U.S. Pat. Nos. 5,396,472 and 5,524,100, both issued to present co-inventor Paffenholz, assigned to the assignee of the present invention, and entitled "Method for Deriving Water Bottom Reflectivity in Dual Sensor Se:-smic Surveys", describe a method which allows the determination of the water bottom reflectivity directly from the production Dual Sensor seismic data rather than from additional calibration data, and describe the advantages of using this method over the prior art. The advantages include deriving water bottom reflectivity from production data without relying on the ratio of the first breaks and without being affected by clipped first signals. A third advantage is in providing a method of combining trace data to eliminate peg-leg reverberations. However, while the operator used in the Paffenholz patents acknowledges the existence of source and receiver side reverberations, it is assumed that the pertinent parameters, water bottom reflectivity and water depth, are similar at the source and the receiver locations. This does not yield a surface consistent map of reflectivities.